1. Field of the Invention
The present invention relates to transparencies for overhead projection. More particularly, the present invention relates to a method for producing transparencies for overhead projection having high color density images wherein the image is formed by image-wise exposing a transparent support having a front surface and back surface coated with an imaging material comprising rupturable microcapsules containing, as an internal phase, a photosensitive composition, a photoinitiator and a color forming agent. Upon rupture of the capsules, the color forming agent on the front surface is image-wise developed and transferred to a developer on a first developer sheet to form an image thereon, and, simultaneously, the color forming agent on the back surface is developed and image-wise transferred to a developer on a second developer sheet to form a mirror image thereon. The two developer sheets containing the image and the mirror image are assembled in developer-to-developer contact and adhered to each other with the image and mirror image in registration to form a transparency having a high color density image.
2. Description of the Prior Art
Photosensitive imaging systems employing microencapsulated radiation-sensitive compositions (also known as cylithographic imaging systems) are the subject of commonly assigned U.S. Pat. Nos. 4,399,209, 4,772,541 and 4,842,976 to The Mead Corporation. These imaging systems are characterized in that an imaging sheet including a layer of microcapsules containing a photosensitive composition in the internal phase is image-wise exposed to actinic radiation. In the most typical embodiments, the photosensitive composition is a photopolymerizable composition including a polyethylenically unsaturated compound and a photoinitiator and is encapsulated with a color precursor. Exposure image-wise hardens the internal phase of the microcapsules. Following exposure, the imaging sheet is subjected to a uniform rupturing force by passing the sheet in contact with a developer sheet through the nip between a pair of pressure rollers whereupon the color precursor is transferred to the developer sheet where it reacts to form an image.
Research Disclosure No. 28246 published in October, 1987 discloses a process for improving color density in transparencies wherein two separate imaging sheets having photosensitive microcapsules containing an internal phase of a photosensitive composition and a color former are image-wise exposed to actinic radiation while the microcapsule layers of the imaging sheets are adjacent to each other so that the imaging sheets are in a mirror relationship. A developer sheet having a developer layer on each surface is placed between the two imaging sheets and the three sheets are then subjected to pressure to rupture the capsules which are in contact with the developer sheet. The color former is transferred from the ruptured capsule to the developer material forming a visible image on both sides of the developer sheet, the images being mirror images of each other.